Keywords:

Introduction

Single-walled carbon nanotubes (SWCNTs) are theoretically expected to display outstanding
mechanical strength, chemical inertness, and excellent thermal and electrical conductivities
[1,2]. However, as-prepared SWCNTs contain a large amount of impurities such as small-sized
catalytic metal particles and carbonaceous materials [3,4]. They also have difficulty in efficient dispersion to display maximum enhanced properties.
Because of their strong intrinsic lateral van der Waals attraction, SWCNTs form bundles
that are strictly entangled (http://hnt.hanwha.co.kr/webcite). Thus, the preparation and purification of SWCNTs are equally important for manufacturing
efficiencies in practice. There are still a few fundamental issues needed to be resolved
first before developing applications. They are related to: (i) cost-effective synthesis
with high purity, and (ii) easy purification without or less damaging SWCNTs. Many
processes for the synthesis of SWCNTs have been reported [4]. However, as-prepared SWCNTs contain more than 60–70 wt% of impurities regardless
what method is used (http://hnt.hanwha.co.kr/webcite). Aside from developing viable SWCNT production on an industrial scale, the purification
and functionalization of SWCNTs continue to be important in nanomaterial research
and development efforts, thus, it would be of practical interest to be able to develop
a scalable, one-pot process for purification and functionalization at the same time
in a non-destructive mild medium. Hence, many attempts to purify SWCNTs have been
reported by using oxidation in nitric acid [5], burning in air [6], using steam [7], etc. However, some reports have pointed out that significant damage in such harsh
conditions has apparently occurred on the sidewall of SWCNTs such as sidewall opening,
breaking, etc. [8-10]. Thus, the purification and functionalization without or little damage on SWCNT framework
are prerequisites to maintain and transfer its outstanding properties to corresponding
matrix as nanoscale additives. As a result, the maximum effective aspect ratio, which
is largely determined by the state of dispersion, could be achieved. In addition,
the chemical modification of SWCNTs, which is able to diminish lateral interaction
between SWCNTs and also to improve chemical affinity between SWCNT and matrix, would
be a viable approach to help efficient dispersion.

We have developed the purification of SWCNTs in a mild and non-destructive medium
in PPA. Specifically, commercial grade PPA (83% P2O5 assay) with additional amount of phosphorous pentoxide (P2O5) medium was optimized condition for “direct” Friedel–Crafts acylation reaction using
a carboxylic acid instead of the corresponding acid chloride [11]. The medium appears to be an ideal system to exploit both purification and functionalization
in a one-pot process. The PPA reaction medium, which has moderately acidic and viscous
characteristics, is expected to play two important roles. Its acidic nature could
protonate to promote de-bundling of SWCNTs and also to decompose pre-existing carbonaceous
impurities and catalytic residues. Its viscous nature would help to impede reaggregation
of SWCNTs after their dispersion. The reaction condition in PPA/P2O5 medium at 130 °C has been utilized in both the polymerization of phenoxybenzoic acids
[11] and functionalization of closely related carbon materials such as vapor-grown multi-walled
carbon nanofibers (MWCNFs) [12,13] and multi-walled carbon nanotubes (MWCNTs) [14-18].

In this work, we prepared an “amide” model compound, 4-(2,4,6-trimethylphenoxy)benzamide
(TMPBA), which was treated with SWCNTs in PPA/P2O5medium. The covalent attachment of TMPBA onto the surface of SWCNTs was studied by
elemental analysis (EA), Fourier-transform infrared spectroscopy (FT-IR), Raman spectroscopy,
and thermogravimatric analysis (TGA). In addition, the morphology of functionalized
SWCNTs was verified by scanning electron microscopy (SEM) and transmission electron
microscopy (TEM).

Experimental

Materials

In this study, all reagents and solvents were purchased from Aldrich Chemical Inc.
and used as received, unless otherwise mentioned. The 4-(2,4,6-trimethylphenoxy)benzamide
was synthesized following the procedure described in a literature and its melting
point was 236–238 °C [19]. Single-walled carbon nanotubes (SWCNTs, 30–40 wt% purity) were obtained from Hanwha
Nanotech Co., LTD, Seoul, Korea (http://hnt.hanwha.co.kr/webcite).

Instrumentation

Fourier-transform infrared (FT-IR) spectra were recorded on a Jasco FT-IR 480 Plus
spectrophotometer. Solid samples were imbedded in KBr disks. Elemental analysis (EA)
was performed by using a CE Instruments EA1110. The melting points (mp) were determined
on a Mel-Temp melting point apparatus and are uncorrected. Thermogravimetric analysis
(TGA) was conducted both in air and nitrogen atmospheres with a heating rate of 10
°C/min using a Perkin–Elmer TGA7. The field emission scanning electron microscopy
(FE-SEM) used in this work was a LEO 1530FE. A FEI Tecnai G2 F30 S-Twin was used for
the field emission transmission electron microscope (FE-TEM) study.

PPA Treatment of SWCNTs at 130 °C

In a 250 mL resin flask equipped with a high-torque mechanical stirrer, nitrogen inlet
and outlet, as-received SWCNTs (1.0 g) and PPA (50 g, 83% P2O5assay) were placed and stirred under dry nitrogen atmosphere at 130 °C for 48 h. After
cooling to room temperature, water was added into the flask. Purified SWCNTs were
precipitated as black powder and collected by suction filtration. The SWCNTs were
Soxhlet-extracted with water for 3 days to completely remove any residual PPA, and
then with methanol for three more days to remove any organo-soluble low molar mass
impurities. Finally, the sample was freeze-dried under reduced pressure (0.5 mmHg)
for 72 h.

Functionalization of SWCNTs with 4-(2,4,6-Trimethylphenoxy)benzamide (TMPBA)

Results and Discussion

Without purification, most of the as-prepared SWCNTs contain approximately 60–70 wt%
of impurities such as carbonaceous fragments, amorphous carbons, and small amount
of graphite and metal catalysts (http://hnt.hanwha.co.kr/webcite). Hence, together with the development of efficient manufacturing to minimize persistent
impurities, the viable purification of prepared SWCNTs is equally important approach
in the field of SWCNT research area. Unlike MWCNTs or MWCNFs, SWCNTs consist of single
graphene layer rolled into tubes. SWCNTs are vulnerable to be damaged on their frameworks
and thus, they will loose their outstanding properties. Thus, as-received SWCNTs with
30–40 wt% purity treated to remove impurities in a much less corrosive medium in PPA
at 130 °C for 48 h. As described in the “Experimental” section, the PPA treated SWCNTs
were recovered and rigorously worked up.

The SEM image obtained from as-received SWCNTs shows bulky particle agglomerates (Fig.
1a), while those impurities have practically disappeared from the PPA treated SWCNTs
(Fig. 1b). The PPA treated SWCNT bundles appear much coarser than the as-received ones, and
thus the average diameter of the PPA treated SWCNT bundles is approximately four times
of that of as-received ones. This is simply because carbonaceous impurities coated
on the surface of the as-received SWCNT bundles are cleaned up, rendering the surfaces
more active (Fig. 1a, inset). The PPA treated SWCNT bundles are able to make better lateral contact each
other and forms larger bundles. Apparently, the surface of PPA treated SWCNT bundles
is seamless and smooth, whereas the surface of as-received SWCNT bundles is furry
and rough due to the impurities. It could be an indication that the larger bundle
thickness implies the higher purity of SWCNTs.

The TEM image of the as-received SWCNTs shows that there are large portions of carbonaceous
and metallic impurities (Fig. 1c). On the other hand, the PPA treated SWCNTs show that most of carbonaceous impurities
have been removed, but some entrapped metallic particles are still present in stable
spherical crystalline phases (Fig. 1d, arrows). Since PPA is not as corrosive as superacids, the entrapped metallic particles
could not be removed without causing the sidewall opening and breaking of the stable
crystalline carbon particles. This implies that PPA can selectively destroy the amorphous
carbons. Unlike SWCNTs treated in hydrochloric acid and nitric acid/sulfuric acid
treatments [20-22], there were no broken SWCNTs in bundles observed in this study. On the basis of these
observations, PPA is indeed a mild and much less destructive medium for the purification
of commercial grade SWCNTs and thus, SWCNTs could preserve their structural integrity.

The efficient functionalization of SWCNTs in the same purification medium might be
the most important progress, since it could allow a one-pot process. For the “direct”
Friedel–Crafts acylation reaction of benzamide instead of benzoic acid chloride, 4-(2,4,6-trimethylphenoxy)benzamide
(TMPBA) was prepared by two step reaction sequences. It was synthesized via aromatic
nucleophilic substitution reaction between 2,4,6-trimethylphenol and 4-fluorobenzonitrile
to give 4-(2,4,6-trimethylphenoxy)benzonitrile, followed by acidic hydrolysis to give
overall good yield. The “direct” Friedel–Crafts acylation reaction between TMPBA and
SWCNTs was carried out to afford TMPBA grafted SWCNTs (TMPBA-g-SWCNT) in PPA/P2O5medium at 130 °C (Fig. 2a). After the reaction mixture had been precipitated in water, the collected product
was completely worked up by Soxhlet-extraction with water for 3 days to completely
remove residual PPA. An additional Soxhlet-extraction was conducted with methanol
for 3 days to ensure the complete removal of any unreacted TMPBA and low molar mass
impurities.

The SEM image of TMPBA-g-SWCNT shows that the surface of SWCNTs is apparently decorated
with covalently bonded moieties (Fig. 2b). Since the expected product should contain carbonyl groups (Fig. 2a), Fourier-transform infrared (FT-IR) spectroscopy was used to monitor the aromatic
ketone υ(C=O) band. The FT-IR spectrum of TMPBA-g-SWCNT shows keto-carbonyl stretching
peak at 1648 cm−1 (Fig. 2c). In addition, the primary amine bands of TMPBA at 3215 and 3386 cm−1 have disappeared in the spectrum from TMPBA-g-SWCNT. The result strongly implies
that covalent attachment of TMPBA onto the surface of SWCNT to afford TMPBA-g-SWCNT.
Furthermore, FT-Raman spectra were taken from PPA treated SWCNTs and TMPBA-g-SWCNT
with 46-mW argon-ion laser (1064 nm) as the excitation source (Fig. 2d). The radial breathing mode (RBM), which appears in low frequency, is a powerful
indicator to determine the nanotube diameters [23]. The RBM frequencies of as-received SWCNTs were 83.36–160.50 cm−1. The values correspond SWCNT diameters of 1.39–2.68 nm by equation, ωRBM = 223.75/dt (ωRBM is the RBM frequency in cm−1; dt is the SWCNT diameter in nm) [24]. In comparison with PPA treated SWCNTs and TMPBA-g-SWCNT, the RBM frequencies of
TMPBA-g-SWCNT were almost identical at 85.29 and 162.43 cm−1 (Fig. 2d). The diameter values, which correspond well to SWCNT diameters of 1.37–2.62 nm.
It is a strong indication that the framework of SWCNTs was not damaged. The ratio
of D/G-band intensity (ID/IG) depends on the SWCNT content. The D-band found near 1275 cm−1 is used to evaluate the defect density present in the tubular wall structure and
the G-band in the 1550–1600 cm−1 region of spectrum is due to the tangential C–C stretching of SWCNT carbon atoms
[25]. The ID/IG value of the TMPBA-g-SWCNT was 1.3, which was much lower than 5.0 of the PPA treated
SWCNTs (Fig. 2d). The result indicates that SWCNTs in TMPBA-g-SWCNT are further purified during
the functionalization in PPA/P2O5 reaction medium. On the basis of combined results from FT-IR and Raman spectra, it
could be tentatively concluded that TMPBA had been attached to electron deficient
SWCNTs via “direct” Friedel–Crafts acylation reaction to give TMPBA-g-SWCNT and SWCNTs
could be further purified in the reaction medium.

A TEM image is provided to further confirm covalent grafting of TMPBA onto SWCNT bundles
(Fig. 3a). Clear stripes in the inner part of SWCNT bundle represent that SWCNT frameworks
are not damaged. The organics coated on the surface of SWCNT bundle are TMPBA moieties,
which are uniformly decorated on the outer surface.

Figure 3.aTEM image of TMPBA-g-SWCNT bundle;bTGA thermograms obtained with heating rate of 10 °C/min in air; andcproposed mechanism of functionalization of SWCNT with benzamide functional group

To obtain char yield and thermooxidative stability, the samples were heated to 800
°C with ramping rate of 10 °C/min under air atmosphere during TGA runs (Fig. 3b). The TGA thermogram of as-received SWCNT shows that the thermooxidative weight
loss occurred in the broad range of 369–593 °C with approximately 57.1% of char yield
at 800 °C (Fig. 3b). The residue at 800 °C in air is expected to be metallic impurities and stable
carbonaceous fragments. The PPA treated SWCNTs displayed the weight loss in the range
348–607 °C with only 8.1% of char yield at 800 °C (Fig. 3b). The value was 49 wt% less than that of as-received SWCNTs. The result suggests
that the heavy weight metallic impurities and most of the carbonaceous fragments,
which are located at outside of SWCNTs, could almost completely be eliminated by PPA
treatment. On the other hand, stable crystalline carbon particles are unaffected by
PPA. The thermooxidative stability of TMPBA-g-SWCNT was the best among the samples.
It showed the temperature at which 5 wt% weight loss (Td5%) had occurred at 420 °C. The Td5%’s of as-received SWCNTs and PPA treated SWCNTs were the same at 369 °C, which was
51 °C lower than that of TMPBA-g-SWCNT. The weight loss of as-received SWCNTs was
gradually started just above 100 °C, indicating that they contained some amount of
volatile impurities. The enhanced thermooxidative stability of TMPBA-g-SWCNT could
due to defective sp3C–H and sp2C–H sites were substituted by aromatic TMPBA. The defects are presumably attributable
to the hydrocarbons, which are used as the major components in the feedstock for SWCNT
productions [26,27]. The defects would provide primary sites for “direct” Friedel–Crafts acylation reaction.
On the basis of the foregoing rationalization, we have recently reported for the first
time that “direct” functionalization and grafting onto as-received vapor-grown MWCNFs
are very effective in PPA/P2O5 medium [12-18]. However, we believe there must be other type of chemical reaction(s) between SWCNT
and carbonium ion to heavily and uniformly functionalize entire SWCNT (see Fig. 2b) [19,28]. The proposed functionalization mechanism of TMPBA onto SWCNTs is presented in Fig.
3c. The mechanism involves that acylium ions are generated directly from benzamide
groups. These ions attack SWCNTs and attach to their surfaces. From combined results,
it is fair to say PPA is indeed less destructive to remove the undesired impurities
from commercial grade SWCNTs. PPA with additional P2O5 is also an efficient medium for covalent attachment of TMPBA onto SWCNTs.

Conclusions

The purification of SWCNTs and the covalent attachment of TMPBA onto the surface of
SWCNTs were conducted in a mild and thus less destructive PPA medium. On the basis
of the results, it was confirmed that this medium could efficiently remove persisted
carbonaceous and metallic impurities in commercial grade SWCNTs with or without little
damage to their framework. In addition, the amide functionality on TMPBA is versatile
for the covalent functionalization of SWCNTs via a simple one-step “direct” electrophilic
substitution reaction. Thus, our approach points out the convenience of purification
and functionalization of SWCNT to a one-pot manufacturing process.

Acknowledgments

We are grateful to Jeong Hee Lee of Chungbuk National University for obtaining SEM
images. We also thank Asian Office of Aerospace Research and Development (AFOSR-AOARD),
Korea Science and Engineering Foundation (R01-2007-000-10031-0) and Chungbuk National
University for their financial supports of this research.